III.] DIVISION INTO REGIONS. 25 



it is generally defined as consisting of those vertebras, between 

 the lumbar and caudal regions, which are ankylosed together 

 in the adult state to form a single bone. It happens, how- 

 ever, that the number of such vertebrae varies in different 

 individuals of the same or nearly allied species, especially as 

 age advances, when a certain number of the tail vertebras 

 generally become incorporated with the true sacrum. 



A more certain criterion is derived from the fact that some 

 of the anterior vertebrae of the sacral region have distinct 

 additional (pleurapophysial) centres of ossification, between 

 the body and the ilium (see Fig. 6, p. 2 1 ). To these perhaps 

 the term sacral ought properly to be restricted, the remaining 

 ankylosed vertebrae being called pseudo-sacral., as suggested 

 by Gegenbaur. Our knowledge of the development of the 

 sacrum^ in different animals is not sufficient at present to 

 apply this test universally, but it appears probable that two 

 is the most usual number of true sacral vertebrae, as thus 

 defined in the Mammalia. 



5. The Caudal Vertebrce. are those placed behind the 

 sacrum, and terminating the vertebral column. They vary 

 in number greatly, being reduced to 5, 4, or even 3, in a 

 most rudimentary condition, in Man, some Apes and Bats, 

 and being numerous and powerfully developed, with strong 

 and complex processes in many Mammals, especially among 

 the Edentata, Cetacea, and Marsupialia. The highest 

 known number, 46, is possessed by the African Long-tailed 

 Minis. 



